SINGLE-PHASE RECTIFIERS WITH CAPACITIVE FILTER

I. OBJECTIVES

a) To establish the dependence between the type of the rectifier (half-wave and full-wave) and the shape of the rectified voltage.

b) The determination of the effects, which appear when the load resistor and the values of the filtering capacitor are modified, upon the rectified voltage.

c) The deduction of the maximum reverse voltage and the maximum forward current through the rectifying diodes.

II. COMPONENTS AND INSTRUMENTATION

For the experiment you will use an experimental board with a transformer, four rectifying diodes, two electrolytic capacitors, a resistor and a potentiometer. In order to visualise the voltages you will use a dual channel oscilloscope, a voltmeter and an ammeter for the measurement of the average values of the voltage and current.

III. PREPARATION

1.P. SINGLE-PHASE HALF-WAVE RECTIFIER

1.1.P. Single-phasehalf-wave rectifier with resistive load
  • How do the waveforms for vS, vO and iOlook like for the circuit shown in Fig. 4.1.if n=22?
  • What is the average value (dc component) V0 of the output voltage? How is it influenced by the value of the load resistorR1+R2+R3, where R1=800Ω, R2=220Ω şi R3=100Ω?
  • What is the value of the maximum reverse voltage on the rectifying diode?
  • What is the value of the output voltage rippleΔVO (the difference between the maximum and minimum value)
  • For what combination of R1, R2 and R3do you obtain the maximum average load current? What is the maximum instantaneous value of the current through the diode (IFmax)?

Notes: The sinusoidal voltage, which should be rectified in this experiment, is obtained from the transformer, Tr , as you can see in the Fig. 4.1. The voltage source used as rectifier is a floating one. Because in the primary of the transformer is the voltage from the main network be careful not to touch this part of the transformer when it is plugged in, because you can be electrocuted.

1.2.P. Single-phasehalf-wave rectifier with capacitive filter

  • What is the difference between the circuits shown in Fig 4.1. and Fig 4.2?
  • How much does the C capacitor influence the voltages Vs(t), V0(t), i0(t)?
  • How is the DC component of V0modified for the circuit shown in Fig. 4.2. in comparison with the circuit shown in Fig.4.1. both having the same average value of the output current i0?
  • How does the value of the output voltage rippleΔVO vary for the circuit in Fig. 4.2 when compared to the circuit in Fig. 4.1, for the same value of the output current.
  • What is value of the maximum reverse voltage, which might appear on D?
2.P. SINGLE-PHASE FULL-WAVE RECTIFIER
2.1.P. Single-phasefull-wave rectifier with resistive load

All the points below refer to the circuit shown in Fig. 4.3.

  • Which of the four diodes are in conduction for the positive alternance of the vs voltage?
  • How do the waveforms of vS and vO look like?
  • What is the difference between the average value of the output voltage and the one at 1.1.P.?
  • What is the difference between the output voltage ripple when compared to the one obtained at 1.1.P ?

2.2.P. Single-phasefull-wave rectifier with capacitive filter

  • How does the output voltage look likes for the circuit in Fig.4.4 (working in open circuit)?
  • Does the waveform of the vO voltage modify if the current is increased to 50mA? If so, how?

Which should be the value of the filtering capacitor (100μF or 1000μF) in order to obtain a higher average value of the output voltage for an output current of 50mA? But for an output current of 0mA?

IV. EXPLORATIONS AND RESULTS

1. SINGLE-PHASE HALF-WAVE RECTIFIER
1.1 Single-phasehalf-wave rectifier with resistive load

Exploration

Build the circuit shown in Fig. 4.1.

  • With the dual channel oscilloscope, calibrated, set on Y-t mode, you will visualise vS and vO. With the dc miliammeter set on 200mAyou will measure the dc component of IO (load current) and with a dc voltmeter you will measure the dc component of vO (output voltage) of the rectifier.
  • With a dc voltmeter you will measure the value of the dc component of VO.
  • For 3 configurations of the load resistors R1, R2 and R3 you will visualize vO(t), measure the dc component of vO and the output voltage rippleΔVO.

Results

  • vS(t) and vO(t) for the values obtained for I0[mA] at 1.1.E.
  • How would you explain the shape of the voltage vO for IO=0mA ?
  • Is there any difference for the voltages at the previous point considering the increase of I0? How can you explain this?
  • Write the values of IO, VOand ΔVOin Table 4.1.
  • Does the value of vOand ΔVOmodifies if IO modifies? Why?
  • What is the value of the maximum reverse voltage, which can be handled by the diode D? The voltage on D is the difference between vS and vO.
  • Which is the value of the maximum constant current through D, IFmax ?

Table 4.1

/ RL [Ω]
C [F] / R1+R2+R3 / R2+R3 / R3
0
IO [V] / 100
1000
0
VO [V] / 100
1000
0
ΔVO[V] / 100
1000

1.2 Single-phasehalf-wave rectifier with capacitive filter

Exploration

Build the circuit shown in Fig. 4.2.

a) C=100μF

The experiment is done in the same way as for the experiment 1.1.

b) C=1000μF

The experiment is done in the same way as for the experiment 1.1.

Results

a) C=100μF

  • vS(t) and vO(t) for the values obtained for I0at 1.1.E.
  • How would you explain the shape of v0 for I0=0mA?
  • How does the increase of IO influence the shape of vO? Why?
  • Write the values of IO,VO andΔVO in Table 4.1.
  • For what value of IO does a maximum reverse voltage appear on D? What is its value? (you can use the waveforms vO and vS)
  • What is the relation between the maximum reverse voltage of the D and the amplitude of vS?

b) C=1000μF.

  • vS(t) and vO(t) for the values obtained for I0 at 1.1.E.
  • How would you explain the shape of vO for I0=0mA?
  • Write the values of IO,VO and ΔVOin Table 4.1.
  • What is the maximum value of the reverse voltage, which might appear across the diode D?
  • The waveforms for VRt and I0 for P=0 (io= vRt/Rt ). What is the value of the maximum forward current through the diode D, IFmax?
  • For what type of rectifier (with or without capacitive filter) is the diode D much more solicitated from the point of view of the maximum direct current?
  • For each value of I0 from the table 4.1. compare the waveforms of vO obtained at the results of 1.2.a and 1.2.b. In which way is vO influenced by the value of the filtering capacitor?
  • How would you explain the big qualitative difference between the forms of ripple of vo(t) obtained at the results of 1.1 and 1.2 ?
  • Using the values from table 4.1. how do the output characteristics of the half-wave rectifier look like vO(IO) in the next situations:

-rectifier with a filter;

-rectifier with a capacitive filter: - C=100μF

- C=1000μF

Draw these two characteristics on the same diagram.

  • Compare these diagrams from the point of view of the load current and the value of the filtering capacitor.
2. SINGLE-PHASE FULL-WAVE RECTIFIER
2.1 Single-phasefull-wave rectifier with resistive load

Exploration

Build the circuit shown in Fig. 4.3.

  • vO is visualised with the calibrated oscilloscope, which is set on Y-t mode.
  • The dc component of vO is measured using the dc voltmeter, and the dc component of IO is measured with the dc miliammeter.
  • vO(t) is visualised, dc component VO, dc component IOand te ripple ΔVOis measured for 3 combinations of the load resistors.

Results

You will draw:

  • The Table 4.2 identically with the Table 4.1 where you will write the data for the single-phase full-wave rectifier.
  • The waveforms of v0 for the values of I0 at 2.1.E.

How would you explain the shape of vO(t)? Which is the difference between this one and the one obtained at the results from 1.1 ?

2.2 Single-phasefull-wave rectifier with capacitive filter

Exploration

Build the circuit shown in Fig. 4.4.

a) C=100μF

The experiment is done in the same way as for the experiment 2.1.

b) C=1000μF

The experiment is done in the same way as for the experiment 2.1.

Results

a)C=100μF

-The waveforms for vO for the values of IO obtained at experiment 2.1

-Why the shape of the output voltage is different compared with the one obtained in the experiment 2.1?

-How does the shape of vo influence the value of Ioobtained at experiment 2.1 ?

-Write the values for IO, VO andΔVO in Table 4.2.

b)C=1000μF

-The waveforms for vO for the values of IO obtained at experiment 2.1

-For the same value of IO compare the shape of the output voltage with the one obtained at paragraph a)

-Write the values IO, VO and ΔVO in Table 4.2.

-Using the values from Table4.2 draw the output characteristics of the full – wave rectifier in the next situations:

-rectifier with a filter

-rectifier with a capacitive filter: C=100μF; C=1000μF

-Compare these diagrams from the point of view of the load current and the value of the filtering capacitor.

-Which type of rectifier (half or full – wave) supplies at the output a bigger dc voltage for the same values of the input voltage (vs), load current and the filtering capacitance (C)?

-If you consider the rectifier as a voltage source, which of the six situations studied so far is the most appropriate to be considered as an ideal voltage source?

-Bring a few criterions based on which you could compare the half – wave rectifier with the full – wave rectifier. According to these criterions, which are the advantages and the drawbacks of each rectifier?

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